Water lentil utilization

The project “LemnaGas” investigated an alternative to the production of biomass for biogas plants. It involved the development of a process with largely closed material cycles that can be used to produce biomass for biogas production in an aquaculture facility. Areas on which the cultivation of crops is not possible or uneconomical, such as fallow land, areas with low rainfall or poor water retention capacity of the soil, landfill areas and also large roofs, can be used for the production of biomass. A further aim was to develop an overall concept for a largely self-sufficient plant consisting of a photobioreactor (cultivation of duckweed/cyanobacteria), a biogas plant (biogas production) and a combined heat and power plant (conversion of the gas into electrical and thermal energy). The data obtained in the laboratory was to be used to evaluate the achievable (energy, land, water, nutrient) efficiency of the overall concept and the opportunities and potential risks of the technology. The project was carried out jointly with the Biomass Research Center Leipzig. The DBFZ investigated the possibilities of separating, preserving and storing the biomass from duckweed/cyanobacteria and converting it into biogas. During the project, the parameters for the cultivation of duckweed were determined. The cultivation system (PhytoLux) was successfully used for this purpose. Optimal parameters are: Temperature of 25 °C, a photon flux density between 60-150 µmol/m²s, a combination of white, red (660nm) and blue (470nm) light and a pH value of 6.0. pH regulation is achieved by dosing CO2 in the form of exhaust air or CO2 gas. Suitable cyanobacteria for co-cultivation are Calothrix species and Anabaena cylindrica. Under optimal culture conditions (PhytoLux), a doubling rate of between 2.8 and 3.0 days could be achieved with Lemna minor, for example. With continuous cultivation in a flat bed reactor, a doubling rate of 3.5 days was achieved with Lemna minor and 5.0 days with Spirodela polyrhiza. At a constant duckweed density of 160 g/m² (doubling time 3.5 days), an annual yield of 166.9 t FM/ha was achieved. With a feasible duckweed density of 500 g/m², a yield of 608 t FM/ha would be possible with a doubling time of 3 days, or a yield of 365 t/ha with a doubling time of 5 days. In Germany, outdoor cultivation is only realistic for 6 months, under tropical conditions all year round. An energy assessment was carried out for year-round cultivation in the greenhouse. This is only feasible if sufficient waste heat is available. In order to be able to estimate and calculate the exact energy requirement, it is essential to operate a test greenhouse.

Project partner: DBFZ gGmbH
Duration: 01.08.2014 – 31.07.2017
Funding body: FNR e.V. on behalf of the BMEL